I. Field of the Invention
The invention relates to telephone central offices which utilize No. 5 crossbar switching equipment. In particular it relates to providing said switching equipment with the capability of routing long distance calls from telephone company customers to preselected long distance carriers. Those of ordinary skill in the art would be those who are familiar with the operation of the No. 5 crossbar switching system.
II. Description of the Prior Art
Most local telephone service in the United States of America is provided by a group of regional telephone companies that were formerly subsidiaries of A.T.&T. These telephone companies are required to provide "equal access" for their customers to long-distance carriers other than A.T.&T. Equal access requires that a customer be able to choose a primary long distance carrier and dial long distance calls directly rather than placing a call to the long distance carrier and using special codes in order to place the call. For example, the familiar 1+ dialing that is available with A.T.&T. would be available with another long distance carrier.
The burden has been placed upon the regional telephone companies to develop switching techniques to implement this "equal access" service. In the past all long distance calls were automatically sent through the A.T.&T. Bell System. Thus, central office equipment was originally designed to efficiently and quickly transfer all long distance calls into the A.T.&T. long distance trunk lines.
There are three basic types of switching equipment currently in use throughout the United States. These systems are step-by-step, No. 5 crossbar and electronic switching systems (ESS). The No. 5 crossbar system is an electromechanical system which is relatively prevalent in medium size central offices. There are several hundred of these systems nationwide which have a combined value in the millions of dollars. Unfortunately these systems are not suited for the providing of equal access long distance service.
In order for the regional telephone companies to effectively implement equal access, technical specifications were agreed upon by the telephone companies and the long distance carriers. These specifications are referred to as "feature group D switched access service". These specifications included the requirements that the switching equipment be capable of handling 16 to 20 digits, that the calling number be transmitted to the long distance carrier first and the called number second, and that additional indentifying digits be added to the normal stream of digits. Also, the switching equipment pursuant to these specifications must record the precise time that the long distance carrier's trunk is seized and the time it is released. These requirements can be met by the electronic switching equipment offices through changes in the software programs in these machines.
However, it has been found that electromechanical switching systems, especially No. 5 crossbar systems, cannot be brought into conformity with the "feature group D" specifications. For instance, the registers on a No. 5 crossbar switching system can accomodate no more than 11 digits. Also, the switching system is specifically designed to pass long distance calls through the system and to their destination before the equipment identifies the party that placed the call. Thus, it is believed by telephone company planners that it would be impossible to provide equal access to telephone calls passing through No. 5 crossbar switches to all long distance carriers.
Since the law mandates that regional telephone companies fully comply with all of the requirements of providing equal access, it appeared necessary to convert all No. 5 crossbar switching systems to ESS systems or to classify all of the customer lines coming into the office by which long distance carrier the customer has selected. The cost of replacing No. 5 crossbar switching systems is over $3 million per central office. The cost nationwide would be in the hundreds of millions of dollars. The incurring of such costs to replace equipment which is functioning well is not justified by projected consumer savings obtained through the equal access to all long distance carriers.
The second alternative of classifying customers by long distance carrier provides a no better result. A typical central office may have 18,000 customers. In order to classify customers, all 18,000 lines would have to be disconnected and reattached on the central office frames based on long distance carrier preference. Such a job would be very complex and very expensive. Also, even if all customers are classified by long distance carrier there would be no way to permit a customer to place a call via a carrier other than the single carrier to which he had originally elected.
The basic problem encountered in providing the No. 5 crossbar with equal access capabilities is that it was not designed to accomodate more than one long distance carrier. It was specifically designed to facilitate traffic through the old Bell System. The underlying theory of the No. 5 crossbar is that a call is routed through the switch in the fastest time possible pursuant to a precision timing pattern. Each component of the switch must exactly synchronize with each other component. Any modification of a switch component affects the entire switching mechanism and thereby diminish the switches' effectiveness in switching both local and long distance calls. This loss of effectiveness is unacceptable.
It is an object of the present invention to provide a means of incorporating equal access capabilities into a central office equipped with a No. 5 crossbar switch. It is another object of the invention to provide said equal access without affecting the underlying principles of the No. 5 crossbar switch. A further object of the invention is to provide equal access to customers served by central offices having a No. 5 crossbar switch in a cost efficient manner which does not affect the quality of their telephone service. A final objective of the invention is to provide equal access to customers served by central offices containing No. 5 crossbar switches through a means which does not adversely affect the rest of the telephone system.